1. Field of the Invention
This invention relates to a water-absorbent resin having an improved dispersibility in water and an improved water-absorbency, and a process for producing the water absorbent resin. More particularly, it relates to a process for producing a water-absorbent resin having an improved water-absorbency and an improved water-dispersibility which comprises crosslinking a water-absorbent resin comprising a carboxylate as a constituent of the resin with a crosslinking agent having at least two functional groups in the presence of water in an inert solvent.
2. Description of Prior Art
Water absorbent resins are used in the field of sanitation as menstrual articles, diaper, disposable house-cloth and the like and in the field of agriculture and horticulture as water retentive materials. Further, they are useful in other various fields such as coagulation of sludges, prevention of dew condensation on construction materials, dehydration of oils and so on. As this type of water-absorbent resin, there are known crosslinked carboxymethyl cellulose, partially crosslinked polyethylene oxide, crosslinked hydrolyzate of starch-acrylonitrile graft copolymer, partially crosslinked polyacrylic acid salt and the like. However, all of them have disadvantages that they have low water-absorbency, and even if the water-absorbency is satisfactory, the water-dispersibility is inferior or the water-absorbing rate is low.
If these disadvantages are overcome, it is clear that the water absorbent resins will be more broadly used in various fields including sanitary articles such as sanitary napkin, paper diaper, underpad, and the like, and it has been desired that improved articles come into the market.
As the water-absorbent resin, there are known various synthetic resins as mentioned above. Particularly, Japanese Patent Application Kokai (Laid-Open) Nos.; 93,716/81; 131,608/81 and 147,806/81 disclose methods for producing water-absorbent resins using as the starting material acrylic acid, which are commercially easily available and are uniform in quality. These water-absorbent resins have an improved water-absorbency even in an aqueous electrolyte solution and an improved stability in the water-absorbed state. However, these water-absorbent resins are still unsatisfactory in dispersibility in water and water-absorbing rate. Moreover, European Patent Application No. 80304099.7 (Publication No. 0036463) and U.S. Pat. No. 4,340,706 disclose that a water-absorbent resin suitable for usages requiring a stability in the fluid-absorbed state for a long period of time or a high water-absorbing rate can be obtained by crosslinking an acrylic acid salt polymer with a crosslinking agent. However, even the resin obtained by said method is still not sufficient in water-dispersibility and water-absorption rate.